With the widespread of mobile broadband applications and the massive amounts of data to be transmitted among end devices, the resource of the radio spectrum has become increasingly scarce, and the technique of creating available bandwidths from the space domain has been developed. Thus, the 3rd Generation Partnership Project (3GPP) has been focusing on the feasibility of supporting device-to-device (D2D) communication in the long term evolution-advanced (LTE-A) standard and the establishment of system requirements. D2D communication is a technique which allows mobile devices to directly communicate with each other by using licensed bands or unlicensed bands (for example, wireless local area networks (WLAN)) in conjunction with heterogeneous networks after proximity discovery under the control of a wireless communication system. The D2D communication technique increases the system spectral efficiency, reduces the transmitting power of each mobile device, and resolves the problem of insufficient spectral resources in a wireless communication system to a certain extent. In addition, the D2D communication technique could satisfy the requirements of proximity communications, such as electronic direct mails and alarm systems, in some commercial applications and during disaster relief.
FIG. 1 is a diagram illustrating an application scenario of D2D communication according to an exemplary embodiment. Referring to FIG. 1, originally, the mobile devices UE1 and UE2 communicate with each other through uplink and downlink communication connections established by a wireless communication system. While the mobile device UE1 slowly moves closer to the mobile device UE2, the communication connection between the mobile devices UE1 and UE2 is switched to D2D communication to reduce the network load (network offloading). While the mobile devices UE1 and UE2 slowly move away from each other, the communication connection between the mobile devices UE1 and UE2 is switched back to the original uplink/downlink communication for D2D communication is not feasible anymore.
The issue of how to maintain the communication quality between two mobile devices when the original uplink/downlink communication is switched to D2D communication is exposed in the application scenario illustrated in FIG. 1. FIG. 2 is a timing diagram of an uplink/downlink communication procedure according to an exemplary embodiment. Referring to FIG. 2, the base station eNB and the mobile device UE have to measure features (including channel quality, pathloss, and timing advance) of the wireless link first. The measurement result of the channel quality allows the base station eNB to optimize the system spectral efficiency when wireless resources are allocated. The measurement result of the pathloss allows the mobile device UE to calculate the transmission power for transmitting data to the base station eNB. The calibration of the timing advance allows the data transmitted by the mobile device UE to be received by the base station eNB at a correct time point even after a propagation delay.
After measuring the features of the wireless link, the base station eNB allocates wireless resources appropriately for the mobile device UE according to the measurement results and transmits the resource allocation result and some control parameters to the mobile device UE. Because the condition of the wireless link may change quickly from time to time due to changes of the transmission environment and the moving speed, the base station dynamically and quickly allocates the wireless resources to the mobile device UE to adapt to any change of the condition of the wireless link. During the data transmission process, LTE extends the hybrid automatic repeat request (HARQ) mechanism adopted in the high speed packet access (HSPA) technology to enhance the data transmission reliability.
HARQ is a technique integrating the feed-forward error correction (FEC) technique and the automatic repeat request (ARQ) technique. The main concept of HARQ is keeping useful information in failed transmission procedures and using such information during the next transmission procedure, so as to increase the success rate of retransmission. HARQ retransmission could be carried out in a synchronous or an asynchronous manner. Regarding the synchronous retransmission, the timing of the HARQ retransmission is pre-determined, and when the receiver fails to receive the data and sends a negative acknowledgement (NACK) back, the transmitter does not wait for an additional control signal and retransmits the data in a pre-scheduled subframe by using the same wireless resources. Contrarily, regarding the asynchronous retransmission, the timing of the HARQ retransmission and the wireless resources used during the retransmission are dynamically changed through control signals.
To maintain the communication quality of mobile devices in D2D communication, the HARQ retransmission mechanism needs to be adopted in D2D communication to enhance the data transmission reliability. However, an existing HARQ program is to be executed only between a base station and a mobile device. Thereby, how to perform HARQ among a base station and two mobile devices which are about to perform D2D communication has become a important issue in the industry.